Hydrogenation of crystalline silicon involves the bonding of hydrogen atoms to crystallographic defects or contamination within the silicon lattice in a way that prevents that defect or contaminant from acting as a recombination site for minority charge carriers. This is known as passivation of the particular recombination site. This is important for semiconductor devices that require long minority carrier lifetimes such as solar cells and particularly where cheap silicon is used that often has poor crystallographic quality and/or purity and therefore needs passivation to bring the quality to acceptable levels for high efficiency solar cells.
Low cost silicon in general has much higher densities of silicon crystallographic defects and/or unwanted impurities. These lower the minority carrier lifetime of the silicon and therefore reduce the efficiencies of solar cells made from such material. Passivation of such defects and contaminants to improve minority carrier lifetimes is therefore an important part of being able to fabricate high efficiency solar cells when using lower quality silicon than that routinely used by the microelectronics industry such as with floatzone (FZ) wafers formed from semiconductor grade silicon.
Currently, without a full understanding of the hydrogenation process and its potential, the designs of commercially manufactured solar cell structures are not ideal to facilitate hydrogenation throughout the cell, and this is reflected in the poor bulk lifetimes for technologies using standard commercial grade p-type wafers.
The ability of hydrogen to move throughout silicon is greatly inhibited by interactions with dopant atoms. For example, in equilibrium in n-type silicon, almost all hydrogen is in the negative charge state (H−) and in p-type silicon almost all hydrogen is in the positive charge state (H+). However hydrogen in these charge states in the respective silicon polarity can lead to strong attraction between the hydrogen atom and the respective dopant atoms, making it difficult for the hydrogen atom to move past such dopant atoms. This can lead to the neutralization of the dopant atoms, and thus the hydrogen can no longer move throughout the silicon. This behaviour of hydrogen in silicon has not been well understood or has been overlooked in the past, with the result that attempts at hydrogenation have been less effective than would have been believed by cell designers.
For example, H+ can interact with ionised boron atoms (B−) to form neutral boron-hydrogen (B—H) complexes. Similarly, H− can interact with ionised phosphorus atoms (P+) to form neutral phosphorus-hydrogen (P—H) complexes.
Boron (B) is a valency 3 element which can be used to dope silicon to produce p-type material when taking on substitutional sites within the silicon lattice. Each such boron atom therefore produces a free “hole”, leaving the boron atom with a fixed negative charge. If atomic hydrogen is directed into such a p-type region and if the hydrogen takes on the positive charge state (H+), strong electrostatic forces exist between the B− and H+ atoms, leading to a high probability that the two will react to form a B—H bond, therefore trapping the hydrogen atom at that location but while simultaneously deactivating the boron atom such that electronically it acts as if it were no longer there.
Conversely, phosphorus (P) is a valency 5 element which can be used to dope silicon to produce n-type material when taking on substitutional sites within the silicon lattice. Each such phosphorus atom therefore produces a free “electron”, leaving the phosphorus atom with a fixed positive charge. If atomic hydrogen is directed into such an n-type region and if the hydrogen takes on the negative charge state (H−), strong electrostatic forces exist between the P+ and H− atoms, leading to a high probability that the two will react to form a P—H bond, therefore trapping the hydrogen atom at that location but while simultaneously deactivating the phosphorus atom such that electronically it acts as if it were no longer there.
The dissociation of the dopant-hydrogen complexes is difficult as even if there is sufficient thermal energy to dissociate the complex (e.g. >150° C.), the coulombic attraction between the dopant atom and the atomic hydrogen (H− for phosphorus and H+ for boron) prevents the escape of the hydrogen atom, and a rapid reformation of the dopant-hydrogen complex is likely.
The dissociation of the dopant-hydrogen complexes is difficult, as even if there is sufficient thermal energy to dissociate the complex (e.g. >150° C.), the coulombic attraction between the dopant atom and the atomic hydrogen (H− for phosphorus and H+ for boron) prevents the escape of the hydrogen atom, and a rapid reformation of the dopant-hydrogen complex is likely.
Minority carrier injection has been observed to enhance the dissociation of dopant-hydrogen complexes. Through minority carrier injection, the dissociation process can occur at much lower temperatures than observed with the absence of minority carrier injection.
In particular, the importance of the minority carrier injection is noted with regard to the associated changes to the charge state of hydrogen. For example, in the dissociation of B—H complexes, during the dissociation process, if the charged hydrogen species (H+) absorbs and electron with a minority carrier, a neutral hydrogen atom (H0) will form. In this state, the hydrogen has high mobility and is unhindered by Coulombic attraction. However H0 is a relatively unstable state and auto-ionises back to H+ in the order of a nanosecond. Subsequently, the H0 is converted back to H+ before it has time to diffuse away from the boron atom and once again forms a B—H complex. However if sufficient electrons are present, the H+ may absorb 2 electrons and form H−, which is a relatively stable charge state and in this charge state, the boron atom repels the H− and it can therefore move throughout the silicon.
It can now be seen that the main reasons for poor hydrogenation in the past include: heavy doping in emitters blocking hydrogen from penetrating deep within the silicon; absence of hydrogen sources from one or both surfaces; aluminium alloyed regions or metal/silicon interfaces acting as sinks; failure to achieve the right charge state for the atoms of hydrogen to facilitate their bonding to certain types of defects and impurities; and no means of trapping of the hydrogen.
While heavy doping might therefore be seen as a disadvantage, understanding the mechanism that can be used to enhance hydrogenation also leads to the possibility of using heavily doped regions to advantage in other ways.
Hydrogen passivation is typically performed on partially fabricated solar cells using predominately a thermal process. For solar cells with screen-printed contacts hydrogenation is often incorporated into the metallization process, however the presence of molten metal and silicon can act as a sink for hydrogen. In addition, subsequent thermal process can often reactivate some of the passivated recombination sites therefore leading to inadequate hydrogen passivation in silicon solar cell modules.
For example, minority carrier injection has been observed to lead to the permanent deactivation of boron-oxygen defects in Czochralski silicon with simultaneous illumination and heating in the range of 70-230 degrees. This can result in stabilized efficiencies which are substantially higher than that which do not receive the stabilization process. However subsequent thermal processes at temperatures above 230 are observed to destabilize the efficiency and the cells are once again subject to the formation of boron-oxygen defects with minority carrier injection. In addition, thermal processes applied on encapsulated cells above 140 degrees Celsius can lead to issues with the EVA encapsulating layers, and processes above 180 degrees can lead to problems with the interconnections in the solar modules.